A prior optical laser using a self coupled semiconductor laser is shown in the Japanese patent laid open publications 133531/82 and 137533/81, in which a semiconductor laser is fed abias current larger than a threshold current, for stimulating the laser, and an output light is focused on a recording medium through a focusing lens. The output light of the laser illuminates the recording medium through the focusing lens, and the feedback beam, reflected by the recording medium, returns to the laser. As the intensity of the output light of the laser depends upon the feedback light, or the reflection factor of the recording medium, the data 0 or 1, stored on the recording medium, is sensed by the intensity of the output light of the laser.
However, a prior self-coupled semiconductor laser optical head has the following disadvantages:
First, a prior optical head is biased larger than the threshold current which stimulates the laser. Thus, the prior laser continuously outputs a strong stimulated emission light. When a pit with a small reflection factor is sensed, the bias current must be close to the threshold current. However, since the laser generates a lot of noise when the bias current is close to the threshold current, if the noise is decreased by increasing the bias current, the output beam will be too strong, and will destroy the data stored on the recording medium.
Secondly, a prior self coupled semiconductor laser optical head has the disadvantage that it is heavy, since it must have a focusing lens. The total weight of the prior optical head is about 60 gr. The recording density and/or the operation speed of an optical disk device using a prior optical head is unfavorably limited.